{"id":111691,"date":"2025-10-16T07:00:00","date_gmt":"2025-10-16T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=111691"},"modified":"2025-10-16T09:06:07","modified_gmt":"2025-10-16T16:06:07","slug":"20251016-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20251016-00\/?p=111691","title":{"rendered":"Using RAII to remedy a defect where not all code paths performed required exit actions"},"content":{"rendered":"

A team asked me to review their pull request that fixed a bug that was caused by failing to perform some required action along all code paths. Here’s a simplified sketch:<\/p>\n

void MySpecialFeature::OnButtonClick()\r\n{\r\n    try {\r\n        auto file = PickFile();\r\n        if (!file) {\r\n            DismissUI();\r\n            return;\r\n        }\r\n\r\n        if (ConfirmAction()) {\r\n            if (m_useAlgorithm1) {\r\n                \/\/ StartAlgorithm1 invokes the lambda when finished.\r\n                StartAlgorithm1(file, [self = shared_from_this()] {\r\n                    self->DismissUI();\r\n                });\r\n            } else {\r\n                RunAlgorithm2(file);\r\n                DismissUI();\r\n            }\r\n        } else { \/\/ this block was missing<\/span>\r\n        \u00a0 <\/span>  DismissUI();                  <\/span>\r\n        }                                 <\/span>\r\n    } catch (...) {\r\n        DismissUI();\r\n    }\r\n}\r\n<\/pre>\n
The problem was the DismissUI()<\/code> call was missing on one of the code paths.<\/p>\n
I suggested that they simplify the code by using an RAII type which will dismiss the UI on all code paths automatically.<\/p>\n
The  Windows Implementation Library<\/a> (commonly known as wil) provides a helper called scope_exit<\/code> which creates and returns an RAII object whose destructor runs the lambda you specify.\u00b9 We can tell it to run a lambda that calls DismissUI()<\/code>.<\/p>\n
Here’s the first try:<\/p>\n
void MySpecialFeature::OnButtonClick()\r\n{\r\n    auto ensureDismiss = wil::scope_exit([&] { DismissUI(); });<\/span>\r\n    try {\r\n        auto file = PickFile();\r\n        if (!file) {\r\n            \/\/ DismissUI();<\/span><\/span>\r\n            return;\r\n        }\r\n\r\n        if (ConfirmAction()) {\r\n            if (m_useAlgorithm1) {\r\n                \/\/ StartAlgorithm1 invokes the lambda when finished.\r\n                StartAlgorithm1(file, [self = shared_from_this()] {\r\n                    ???<\/span>\r\n                });\r\n            } else {\r\n                RunAlgorithm2(file);\r\n                \/\/ DismissUI();<\/span><\/span>\r\n            }\r\n        } \/\/ else {<\/span>      <\/span>\r\n        \u00a0 <\/span>\/\/ DismissUI();<\/span><\/span>\r\n        \/\/ }<\/span>             <\/span>\r\n    } catch (...) {\r\n        \/\/ DismissUI();<\/span><\/span>\r\n    }\r\n}\r\n<\/pre>\n
The ensureDismiss<\/code> is an RAII type created by scope_exit<\/code> that runs the lambda at destruction, which in this case means that it calls DismissUI()<\/code>. The nice thing about C++ destruction of local variables is that it happens when scope ends, whether it be by executing off the end of the block, by an early exit (say, an early return<\/code>), or an exception.<\/p>\n
There is one section marked with question marks, though. We don’t want DismissUI()<\/code> to happen until Algorithm1 finishes its work. How do we delay the destruction of the ensureDismiss<\/code> object?<\/p>\n
Well, we can’t. It will destruct when the scope ends.<\/p>\n
However, the objects created by wil::scope_exit<\/code> are movable. When you move them, the obligation to run the lambda is transferred to the moved-to object. So you can move-capture the ensureDismiss<\/code> into the lambda, and that will transfer the responsibility to call DismissUI()<\/code> into the lambda. Fortunately, the lambda already captures shared_from_this()<\/code>, so there is a shared_ptr<MySpecialFeature><\/code> inside the lambda. We can’t see it, but it’s clear that StartAlgorithm1<\/code> must move its lambda to some safekeeping location so that it can invoke it later. When the lambda moves, the obligation moves with it, and when the final moved-to lambda destructs, the DismissUI()<\/code> will happen.<\/p>\n
void MySpecialFeature::OnButtonClick()\r\n{\r\n    auto ensureDismiss = wil::scope_exit([=<\/span>] { DismissUI(); });\r\n    try {\r\n        auto folder = PickOutputFolder();\r\n        if (!folder) {\r\n            return;\r\n        }\r\n\r\n        if (ConfirmAction()) {\r\n            if (m_useAlgorithm1) {\r\n                \/\/ StartAlgorithm1 invokes the lambda when finished.\r\n                StartAlgorithm1(file, [self = shared_from_this(),\r\n                    ensureDismiss = std::move(ensureDismiss)<\/span>]\r\n                    { });\r\n            } else {\r\n                RunAlgorithm2(file);\r\n            }\r\n        }\r\n    } catch (...) {\r\n    }\r\n}\r\n<\/pre>\n
Note that we changed the capture of the lambda passed to scope_exit<\/code> a value capture, because a reference capture would be capturing references to locals that have destructed. In this case, we don’t capture any locals, so it’s not significant, but it’s good hygiene and avoids embarrasing questions later. (A hidden gotcha is that a value capture captures this<\/code> by value, so there is actually still a reference capture in there, namely a reference to *this<\/code>.)<\/p>\n
In fact, for even better hygiene, you might want to capture the shared_ptr<\/code> into the lambda so that the lifetime promises are more explicit.<\/p>\n
void MySpecialFeature::OnButtonClick()\r\n{\r\n    auto ensureDismiss = wil::scope_exit([self = shared_from_this()<\/span>]\r\n            { self->DismissUI(); });\r\n    try {\r\n        auto folder = PickOutputFolder();\r\n        if (!folder) {\r\n            return;\r\n        }\r\n\r\n        if (ConfirmAction()) {\r\n            if (m_useAlgorithm1) {\r\n                \/\/ StartAlgorithm1 invokes the lambda when finished.\r\n                StartAlgorithm1(file, [\/\/ self = shared_from_this(),<\/span><\/span>\r\n                    ensureDismiss = std::move(ensureDismiss)<\/span>]\r\n                    { });\r\n            } else {\r\n                RunAlgorithm2(file);\r\n            }\r\n        }\r\n    } catch (...) {\r\n    }\r\n}\r\n<\/pre>\n
There’s a little wrinkle here that we can iron out look at next time.<\/p>\n
Bonus chatter<\/b>: The object created by wil::scope_exit<\/code> patterns itself after std::unique_ptr<\/code>, so you can reset()<\/code> it to go through its destructor cleanup early, and you can release()<\/code> it to say “I changed my mind. Don’t run the lambda at all.”<\/p>\n
\u00b9 A corresponding proposal for the C++ standard library has been around  since 2013<\/a>. (Followed up by N3830<\/a>, N3949<\/a>, N4189<\/a>, p0052r1<\/a>, through p0052r10<\/a>, then became part of N4786<\/a> (Working Draft, C++ Extensions for Library Fundamentals, Version 3), which appears to have been approved in 2024 as an experimental feature.<\/p>\n","protected":false},"excerpt":{"rendered":"
Passing the obligation onward.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-111691","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing"],"acf":[],"blog_post_summary":"
Passing the obligation onward.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111691","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=111691"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111691\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=111691"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=111691"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=111691"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}